10 Penetrometer Techniques in Relation to Soil Compaction and Root Growth A. Glyn Bengough Scottish Crop Research Institute, Dundee, Scotland Donald J. Campbell and Michael F. O’Sullivan Scottish Agricultural College, Edinburgh, Scotland I. INTRODUCTION Soil hardness is the resistance of the soil to deformation, be it by a plant root, the blade of a plow, or the tip of a penetrometer. Hard soils are a major problem in agriculture worldwide; they restrict root growth and seedling emergence, increase the energy costs of tillage, and impose restrictions on the soil management re- gimes that can be used. Penetrometers are used commonly to measure soil strength. If a standard probe and testing procedure is used, penetrometers give an empirical measure of soil strength that enables comparisons between different soils. A penetrometer consists typically of a cylindrical shaft with a conical tip at one end, and a device for measuring force at the other (Fig. 1). Penetration resistance is the force re- quired to push the cone into the soil divided by the cross-sectional area of its base (i.e., a pressure). The American Association of Agricultural Engineers specified a standard penetrometer design that gives a measurement called the cone index (ASAE, 1969). This standard has been adopted widely, but many nonstandard penetrometers are in use. Nonstandard penetrometers and testing procedures are more appropriate for some applications, as long as comparisons are made using the same procedure. The principles behind the testing procedure must be under- stood so that the results can be interpreted sensibly. In this chapter we describe the theory behind the measurement of penetra- tion resistance, and how penetration resistance is related to other soil properties. Copyright © 2000 Marcel Dekker, Inc. We then consider the practical aspects of penetrometer measurements, including the design of the apparatus, the availability of equipment, the measurement pro- cedure, and the interpretation of data. In the final section we discuss how to apply the technique to studies of trafficability, tillage, compaction, and root growth. II. THEORY A. Soil Penetration by Cones Penetration resistance can, in principle, be estimated from the bulk mechanical properties of the soil. Farrell and Greacen (1966) developed a model of soil pene- tration in which penetration resistance consisted of two components: the pressure required to expand a cavity in the soil, and the frictional resistance to the probe. Penetrometer resistance, Q, is given by Eq. 1 (Farrell and Greacen, 1966), includ- ing the effects of adhesion (Bengough, 1992): Q ϭ s(1 ϩ cot a tan d) ϩ c cot a (1) a where s is the stress normal to the cone surface, a is the cone semiangle, d is the angle of soil–metal friction, and c a is the soil–metal adhesion. This equation as- sumes that the soil is homogeneous and isotropic, that the frictional resistance between the penetrometer shaft and the soil is negligible, that the cone angle of the penetrometer is sufficiently small so that no soil-body accumulates in front of the cone, and that the stress is distributed uniformly on the cone surface. The normal stress, s, was equated with the pressure required to expand a cylindrical or spherical cavity in the soil. Expansion of the cavity occurred 378 Bengough et al. Fig. 1 Schematic diagram of a penetrometer showing cone, shaft, and force transducer. Copyright © 2000 Marcel Dekker, Inc. through compression of the soil surrounding the probe. Two distinct zones were identified: a zone of compression with plastic failure surrounding the probe, with a zone of elastic compression immediately outside it (Farrell and Greacen, 1966). Calculating s required measurements of many soil mechanical properties. The value of s was predicted for three soils at different bulk densities and matric po- tentials. For cylindrical soil deformation, s was only 0.25–0.45 of that for spheri- cal deformation. Greacen et al. (1968) suggested that roots and penetrometers with narrow cone angles cause cylindrical soil deformation, while penetrometers with larger cone angles cause spherical deformation. The detailed measurements and calculations required to predict s show that it is much easier to measure penetration resistance than to predict it. One of the major findings of this work was the large contribution of friction to penetration resistance. Friction on a 5Њ semiangle probe accounts for more than 80% of the total penetration resistance (Eq. 1). This has been tested using a penetrometer with a rotating tip (Bengough et al., 1991, 1997). Rotation of the penetrometer tip decreased the resultant component of friction directed along the penetrometer shaft. The measured penetration resistance agreed closely with the predicted resis- tance in a range of soils. When the cone angle exceeds 90Њ Ϫ f, where f is the angle of internal friction of the soil, a cone of soil builds up on the probe tip (Koolen and Kuipers, 1983). This body of soil moves with the probe, so that friction occurs between the soil body and the surrounding soil, instead of between the metal and soil surfaces. Equation 1 can therefore be applied only to probes with relatively narrow cone angles. Penetrometer design, testing procedure, and the effects on penetration re- sistance are considered in Sec. III. B. Effects of Soil Properties on Penetration Resistance Penetration resistance depends on soil type—the distribution of particle sizes and shapes, the clay mineralogy, the amorphous oxide content, the organic matter con- tent, and the chemistry of the soil solution (Gerard, 1965; Byrd and Cassel, 1980; Stitt et al., 1982; Horn, 1984). Within a given soil type, the penetration resistance depends on the bulk density, water content, and structure of the soil. Penetration resistance can be affected by the pretreatment of the soil prior to testing. Hence the penetration of samples that have been dried, sieved, rewetted, and remolded will probably be very different from the penetration resistance of the soil in the field. The purpose of the experiment must therefore be considered carefullybefore the soil is sampled or penetration resistance is measured. Penetration resistance decreases with increasing soil water content, and it increases with increasing bulk density. Gravimetric water content is a useful mea- sure of water status, as matric potential and volumetric water content may change as soil is compressed during penetration (Koolen and Kuipers, 1983). Matric Penetrometer Techniques in Compaction and Root Growth 379 Copyright © 2000 Marcel Dekker, Inc. potential, however, is the mechanistic link to effective stress and hence to soil strength, via the surface tension of water-films holding the soil particles together (Marshall et al., 1996). Water content has little effect on cone resistance in loose soil, but its effect increases with bulk density. The influence of bulk density on cone resistance is greater in dry than in wet soil. Different functions have been proposed to describe these relations (Perumpral, 1983). For a given soil, the sim- plest suitable function is Q ϭ k ϩ k u ϩ k r ϩ k ru (2) 12m3 4m where u m is gravimetric water content, r is dry bulk density, and k 1 k 4 are empirical constants (Ehlers et al., 1983). This relation is applicable widely and is illustrated in Fig. 2, using values of the constants for a loess soil. In some soils, however, the changes in cone resistance with bulk density and water content are not linear: cone resistance changes most rapidly at high bulk densities and low water contents. The linear model (Eq. 2) may still be appropriate if the ranges of bulk density and water content are small or soil variability is high, but other mod- els may be valid more generally (Perumpral, 1983). 380 Bengough et al. Fig. 2 Variation of penetrometer resistance with water content at different bulk densities. (Based on data from Ehlers et al., 1983.) Copyright © 2000 Marcel Dekker, Inc. The relation between soil strength (in this case measured as penetration resistance) and matric potential is known as the soil strength characteristic. The main problem in deriving and applying such empirical relations is that soil strength changes with time, even if bulk density and water content remain constant (Davies, 1985). Soil management practices affect soil structure, changing the con- stants in these empirical relations. At constant water content and bulk density, cone resistance tends to increase with decreasing particle size (Ball and O’Sullivan, 1982; Horn, 1984). Thus a clay will have a larger penetration resistance for a given gravimetric water content than a sand. This is due to the greater effective stress associated with the lower matric potential in the finer textured soil. In general, the decrease in organic matter as- sociated with the intensive cultivation or deforestation of soils is associated with an increase in the gradient of the soil strength characteristic (Mullins et al., 1987). III. PENETROMETER DESIGN Details of a selection of commercially available penetrometers are given in Table 1. Penetrometers can be classified broadly as ‘‘needle’’ type if they have a diameter smaller than about 5 mm. Most needle penetrometers are used for test- ing of soils in the laboratory, though some have been used in the field. Penetrom- eters that are used in the field often have a diameter greater than 10 mm. Many penetrometers have also been designed for specific purposes. Needle penetrometer measurements can be made in the laboratory by attaching a suitable probe to the force transducer of a loading frame designed for material testing. In the following sections, the effects of penetrometer design and testing procedure on penetration resistance measurements are considered. A. Cone Angle and Surface Properties Penetrometer tips are generally cones, although flat-ended cylinders (Groenevelt et al., 1984) and shapes resembling the tips of plant roots (Eavis, 1967) have been used. The shape of the tip determines both the mode of soil deformation and the amount of frictional resistance on the tip. Penetrometer resistance is a minimum at a cone angle of 30Њ (Fig. 3; Gill, 1968; Voorhees et al., 1975; Koolen and Vaan- drager, 1984). Increased cone resistance is associated at small cone angles with the increased component of soil–metal friction and, at large cone angles, with soil compaction in front of the cone (Gill, 1968; Mulqueen et al., 1977). Figure 3, which was derived from measurements made in 67 agricultural fields (Koolen and Vaandrager, 1984) shows the relationship between cone resistance and cone angle for a fixed cone base area. Soil tends to be displaced laterally at small cone angles, whereas the direction of displacement becomes more vertical with increasing cone angles (Gill, 1968; Tollner and Verma, 1984). Lateral soil displacement relates more closely to the mechanics of root growth than does the more axial displace- Penetrometer Techniques in Compaction and Root Growth 381 Copyright © 2000 Marcel Dekker, Inc. ment produced by probes with larger cone angles (Greacen et al., 1968). Con- versely, the load-bearing characteristics of the soil are more closely related to the resistance encountered by larger cone angles. Penetrometers that are available commercially are generally fitted with 30Њ or 60Њ cones, but these can be easily interchanged. The surface roughness of the cone is not an important factor in penetrometer design, as abrasion by soil particles quickly removes any minor irregularities. Lu- brication of the cone decreases penetration resistance by decreasing soil–cone friction and the movement of soil in the axial direction (Gill, 1968; Tollner and Verma, 1984). Use of such a lubricated penetrometer is of questionable advantage, as the mechanics of penetration of a lubricated cone is poorly understood, and the lubricating technology may be difficult to standardize. 382 Bengough et al. Table 1 Suppliers of Some Penetrometers, Force Transducers, and Load Frames Available Commercially Supplier Address Equipment Approximate cost (US$) ELE Inter- national Ltd. In the UK: Eastman Way, Hemel Hempstead, Hertfordshire, HP2 7HB In the USA: 86 Albrecht Drive, P.O. Box 8004, Lake Bluff, Illinois 60044-8004 Field penetrometer with data logger, hand-held. 7500 Soil Test Inc. 2250 Lee Street, Evanston, Illinois 60202, USA Proving ring penetrometer Eijkelkamp P.O. Box 4, 6987ZG Giesbeek, The Netherlands Field penetrometer with data logger, hand-held 8800 Leonard Farnell & Co. Ltd. North Mymms, Hatfield, Hert- fordshire AL9 7SR, UK Simple hand-held pene- trometer with dial gauge. 1000 Ametek Mansfield & Green Division, 8600 Somerset Drive, Largo, Fl 34643, USA Wide range of loading frames and force trans- ducers. Agents also in UK. Pioden Con- trols Ltd. Graham Bell House, Roper Close, Roper Road, Canter- bury, Kent CT2 7EP, UK Force transducers suitable ranges for needle penetrometers. From about 270 Applied Measure- ments Ltd. 3 Titan House, Calleva Park, Aldermaston, Reading, Berkshire, RG7 4QW, UK Force transducers suitable ranges for needle penetrometers From about 225 Inclusion in this list does not constitute any recommendation of the product. Copyright © 2000 Marcel Dekker, Inc. B. Cone Base Diameter In general, the diameter of needle penetrometers is important and must be taken into account when comparing results from different instruments. Diameter is less important when comparing field penetrometers. The diameter of the cone bases range from large field penetrometers (Ͼ10 mm) (Ehlers et al., 1983) to small needle penetrometers (Ͻ0.2 mm) (Groenevelt et al., 1984). Although cone resistance is expressed as a force per unit base area, it tends to increase with decreasing base area (Freitag, 1968). For field penetrometers, the standard of the American Society of Agricultural Engineers (ASAE, 1969) allows cone base areas of 320 mm 2 and 130 mm 2 ,bothwitha30Њ cone angle. A 3% decrease in diameter is allowed for cone wear. In Europe, cones of 100 mm 2 base area are common, but cones with base areas of up to 500 mm 2 have been used. Even in homogeneous soil, penetration resistance can depend on probe di- ameter as soil particles of finite size must be displaced. Diameter dependence is Penetrometer Techniques in Compaction and Root Growth 383 Fig. 3 Variation of penetrometer resistance with cone angle for a fixed cone base area. (From Koolen and Vaandrager, 1984. Reproduced with permission from the Journal of Agricultural Engineering Research.) Copyright © 2000 Marcel Dekker, Inc. most noticeable for very small probes, which may have to displace particles of comparable size. The effect of probe diameter on penetration resistance depends on the soil type, water content, and structure (Whiteley and Dexter, 1981). In remolded soil cores with textures ranging from clay to sand, resistance to a 1 mm probe was typically 45–55% greater than to a 2 mm diameter probe (Whiteley and Dexter, 1981). Other studies found no significant effect of diameter among 1, 2, and 3 mm diameter probes in remolded sandy loam (Barley et al., 1965), be- tween 3.8 and 5.1 mm probes in undisturbed cores (Bradford, 1980), and between 1 and 2 mm probes in both undisturbed clods and remolded soils (Whiteley and Dexter, 1981). There is need for a comprehensive study over a wide range of penetrometer diameters and soil textures. In soils with well-developed structural units, the mechanism of penetration may differ between cones of different sizes. A cone with a small diameter, relative to the size of structural units, may penetrate aggregates or planes of weakness between aggregates, whereas a large cone will tend to deform aggregates (Jamie- son et al., 1988). C. Shaft Diameter The surface area of a penetrometer shaft is directly proportional to its diameter, whereas the force on the penetrometer tip is proportional to the square of the tip diameter. Thus shaft friction is relatively more important for smaller probes, and this has been confirmed by experiment (Barley et al., 1965). To decrease soil– metal shaft friction, a relieved shaft (i.e., a shaft with a diameter 20% smaller than the probe tip) is used commonly. Shaft friction can significantly increase the resistance even to a standard ASAE penetrometer, especially in wet clay (Freitag, 1968; Mulqueen et al., 1977). Freitag (1968) found that increasing the shaft diameter from 9.5 mm to 15.9 mm (the ASAE standard) increased the resistance threefold at 0.3 m depth on a stan- dard 20.3 mm diameter cone. Similarly, Reece and Peca (1981) used a shaft 8 mm in diameter to eliminate the clay–shaft friction on the standard 20.3 mm diame- ter cone. IV. PENETROMETER INSERTION AND MEASUREMENT A. Force Measurement The commonest and most easily interpreted penetrometer results are from mea- suring the resistance to a probe driven into soil at a constant speed. Other designs measure the magnitude or the rate of probe penetration under different constant loads (van Wijk, 1980). In this chapter only penetrometers designed to be used at a constant rate are considered. 384 Bengough et al. Copyright © 2000 Marcel Dekker, Inc. 1. Laboratory Needle Penetrometers To obtain a constant rate of penetration in the laboratory, it is necessary either to drive the probe downward into the soil with some sort of motor (Barley et al., 1965) or to raise the soil sample on a moving platform toward a stationary probe (Eavis, 1967). The movable crosshead of a strength testing machine has a conve- nient drive capable of a wide range of speeds, and can accept force transducers to measure the force resisting penetration (Fig. 4; Callebaut et al., 1985; Bengough et al., 1991). Proving rings, strain gauges, and electronic balances have all been used to measure the force resisting penetration (Barley et al., 1965; Eavis, 1967; Penetrometer Techniques in Compaction and Root Growth 385 Fig. 4 Needle penetrometer attached to a force transducer on a loading frame. Copyright © 2000 Marcel Dekker, Inc. Misra et al., 1986a). The advantage of an electronic balance or force transducer is that the output can be logged using the analog-to-digital converter of a datalogger or personal computer. Proving rings that are too flexible can result in small voids going undetected, as the proving ring expands when unloaded. 2. Field Penetrometers A field penetrometer may be mounted on a rack to allow easy and precise location (Soane, 1973; Billot, 1982). This facilitates measurements on a regular, closely spaced grid. Hand-held penetrometers are more portable, are cheaper, and can be used in inaccessible field sites (Fig. 5). Automatic logging of force is very advantageous, as it is difficult for the operator to record measurements at predefined depths. Analog recording using a 386 Bengough et al. Fig. 5 Field penetrometer with data storage unit. Copyright © 2000 Marcel Dekker, Inc. [...]... I Soil and wheel characteristics Soil Till Res 1 : 207–237 Copyright © 2000 Marcel Dekker, Inc Penetrometer Techniques in Compaction and Root Growth 403 Stitt, R E., D K Cassel, S B Weed, and L A Nelson 1982 Mechanical impedance of tillage pans in Atlantic coastal plains soils and relationships with soil physical, chemical and mineralogical properties Soil Sci Soc Am J 46 : 100 106 Stolzy, L H., and. .. W Holmes, and C W Rose 1996 Soil Physics 3d ed Cambridge: Cambridge University Press McIntyre, D S., and C B Tanner 1959 Anormally distributed soil physical measurements and non-parametric statistics Soil Sci 88 : 133 –137 Misra, R K., A R Dexter, and A M Alston 1986a Penetration of soil aggregates of finite size: I Blunt penetrometer probes Plant Soil 94 : 43 –58 Misra, R K., A R Dexter, and A M Alston... E., R J Morris, and C E Mullins 1988 Effect of subsoiling on physical properties and crop growth on a sandy soil with a naturally compact subsoil In: Proc 11th Int Conf Int Soil Till Res Organization, Vol 2, pp 499 –503 Johnston, C E., R L Jensen, R L Schafer, and A C Bailey 1980 Some soil tool analogs Trans Am Soc Agric Eng 23 : 9 –13 Knight, S J., and D R Freitag 1962 Measurement of soil trafficability... entering compact soils Soil Sci 105 : 297– 310 Taylor, H M., and L F Ratliff 1969 Root elongation rates of cotton and peanuts as a function of soil strength and water content Soil Sci 108 : 113 –119 Threadgill, E D 1982 Residual tillage effects as determined by cone index Trans Am Soc Agric Eng 25 : 859 – 867 Tollner, E W., and B P Verma 1984 Modified cone penetrometer for measuring soil mechanical impedance... Soft Soils U.S Army Waterways Exp Stn Rep No 3-6 88 Freitag, D R 1968 Penetration tests for soil measurements Trans Am Soc Agric Eng 11 : 750 –753 Freitag, D R., R L Schafer, and R D Wismer 1970 Similitude studies of soil- machine systems Trans Am Soc Agric Eng 13 : 201–213 Gerard, C J 1965 The influence of soil moisture, soil texture, drying conditions and exchangeable cations on soil strength Soil Sci... weight, tire soil contact stress, engine power, and transmission type A dimensional analysis of tire soil and cone soil interaction led to the development of dimensionless mobility numbers for dry, cohesionless sands, and Copyright © 2000 Marcel Dekker, Inc Penetrometer Techniques in Compaction and Root Growth 393 saturated, frictionless clays (Freitag, 1965) The clay and sand mobility numbers N c and N... The untilled soil contained more cracks and biopores that Copyright © 2000 Marcel Dekker, Inc 398 Bengough et al Fig 9 Root elongation rate for peanuts and cotton versus soil penetrometer resistance (Reproduced from H M Taylor and L F Ratliff, Root elongation rates of cotton and peanuts as a function of soil strength and water content Soil Science 108 : 113 –119 (1969) ᭧ by Williams and Wilkins, Baltimore,... Bengough, and G J Ley 1987 Hard-setting soils Soil Use Manag 3 : 79 – 83 Mulqueen, J., J V Stafford, and D W Tanner 1977 Evaluating penetrometers for measuring soil strength J Terramech 14 : 137–151 O’Sullivan, M F., and B C Ball 1982 A comparison of five instruments for measuring soil strength in cultivated and uncultivated cereal seedbeds J Soil Sci 33 : 597– 608 O’Sullivan, M F., J W Dickson, and D J... and D J Campbell 1987 Interpretation and presentation of cone resistance data in tillage and traffic studies J Soil Sci 38 : 137–148 Paul, C L., and J de Vries 1979 Effect of soil water status and strength on trafficability Can J Soil Sci 59 : 313 –324 Paul, C L., and J de Vries 1983 Soil trafficability in spring: 2 Prediction and the effect of subsurface drainage Can J Soil Sci 63 : 27–35 Perumpral, J V... Ball, B C., and M F O’Sullivan 1982 Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments J Soil Sci 33 : 609 – 622 Barley, K P., E L Greacen, and D A Farrell 1965 The influence of soil strength on the penetration of a loam by plant roots Aust J Soil Res 3 : 69 –79 Barraclough, P B., and A H Weir 1988 Effects of a compacted subsoil layer on root and shoot . Netherlands Field penetrometer with data logger, hand-held 8800 Leonard Farnell & Co. Ltd. North Mymms, Hatfield, Hert- fordshire AL9 7SR, UK Simple hand-held pene- trometer with dial gauge. 100 0 Ametek. of soil metal friction, and c a is the soil metal adhesion. This equation as- sumes that the soil is homogeneous and isotropic, that the frictional resistance between the penetrometer shaft and. the soil. Farrell and Greacen (1966) developed a model of soil pene- tration in which penetration resistance consisted of two components: the pressure required to expand a cavity in the soil, and